November 23, 2005

Einstein’s dark energy accelerates the universe

by ssavage

The genius of Albert Einstein, who added a "cosmological constant" to his equation for the expansion of the universe but later retracted it, may be vindicated by new research published today in the journal Astronomy and Astrophysics.

The enigmatic "dark energy" that drives the acceleration of the Universe behaves just like Einstein's famed cosmological constant, according to the Supernova Legacy Survey (SNLS), an international team of researchers in France and Toronto and Victoria in Canada, collaborating with large telescope observers in Oxford, Caltech and Berkeley. Their observations reveal that the dark energy behaves like Einstein's cosmological constant to a precision of 10%.

"The significance is huge," said Professor Ray Carlberg of the Department of Astronomy and Astrophysics at the University of Toronto. "Our observation is at odds with a number of theoretical ideas about the nature of dark energy that predict that it should change as the universe expands, and as far as we can see, it doesn't."

"We have set ourselves a very challenging goal - to distinguish whether the dark energy can be explained by Einstein's cosmological constant or whether a new physical theory is needed." Says Dr Isobel Hook of the University of Oxford, "So far our results are consistent with Einstein's cosmological constant, but the best is still to come. The first year results already represent the largest homogeneous set of distant supernovae, but over the full five years of the survey we will improve our precision more and more. Our goal is a measurement of the nature dark energy that will be a true legacy for years to come."

She added "Before dark energy was being considered, Einstein invented the 'cosmological constant' to make his equations fit with his ideas about the Universe, but later regretted it, calling it his biggest blunder'. Now we know he may have been closer to the truth than he realised."

The Supernova Legacy Survey (SNLS) aims to discover and examine 700 distant supernovae to map out the history of the expansion of the universe. The survey confirms earlier discoveries that the expansion of the universe proceeded more slowly in the past and is speeding up today, apparently driven by some unknown form of energy. Since scientists don't know much about this mysterious new form of energy, they call it "dark energy."

The researchers made their discovery using an innovative, 340-million pixel camera called Megacam, built by the Canada-France-Hawaii Telescope and the French atomic energy agency, Commissariat Ã l'Ã“°nergie Atomique. "Because of its wide field of view -- you can fit four full moons in an image -- it allows us to measure simultaneously, and very precisely, several supernovae, which are rare events," said Pierre Astier, one of the scientists with the Centre national de la recherche scientifique (CNRS) in France.

"Improved observations of distant supernovae are the most immediate way in which we can learn more about the mysterious dark energy," adds Richard Ellis, professor of astronomy at the California Institute of Technology. "This study is a very big step forward in quantity and quality."

Study co-author Saul Perlmutter, a physics professor at the University of California, Berkeley, says the findings kick off a dramatic new generation of cosmology work using supernovae. "The data is more beautiful than we could have imagined 10 years ago -- a real tribute to the instrument builders, the analysis teams and the large scientific vision of the Canadian and French science communities."

The SNLS is a collaborative international effort that uses images from the Canada-France-Hawaii Telescope, a 3.6-metre telescope atop Mauna Kea, a dormant Hawaiian volcano. The current results are based on about 20 nights of data, the first of over nearly 200 nights of observing time for this project. The researchers identify the few dozen bright pixels in the 340 million to find distant supernovae. They acquire spectra using some of the largest telescopes on Earth--the Frederick C. Gillett Gemini North Telescope on Mauna Kea, the Gemini South Telescope on the Cerro Pach³n mountain in the Chilean Andes, the European Southern Observatory Very Large Telescopes (VLT) at the Paranal Observatory in Atacama, Chile, and the Keck telescopes on Mauna Kea.

In the UK the work has been done by Dr Isobel Hook and her student (Justin Bronder) in Oxford. Their focus has been on obtaining spectra with Gemini to measure redshifts and confirm the supernova types. Only certain types of supernovae are useful for cosmology, namely those classed as "Type Ia" which they identify by particular signatures in their spectra.

The "queue" observing mode used at Gemini and VLT is ideal for this project. When they find good supernova candidates from CFHT they send instructions over the internet to the staff at Gemini and VLT, and they take data for them when the weather conditions are right for the program. The instruments used on the Gemini telescopes for this project are the GMOS - the Gemini Multi-object spectrographs - built in the UK (by the UKATC and University of Durham) and Canada.

"Only the world's largest optical telescopes -- with diameters of eight to 10 metres -- are capable of studying distant supernovae in detail by examining the spectrum," said Dr Isobel Hook.

The current paper is based on about one-tenth of the imaging data that will be obtained by the end of the survey. Future results are expected to double or even triple the precision of these findings and conclusively solve several remaining mysteries about the nature of dark energy.